The present invention relates to a point feeder or metering device for metering or dosing powder materials, for example, aluminum oxide (called oxide in the following), to a Hall-Heroult electrolysis cell. The point-feeder includes a vertical tube and a horizontal tube which are joined so that they basically form an L-shaped channel with material inflow at the upper end of the vertical tube and material outflow at the end of the horizontal tube.
Point feeders or metering devices and transport devices which use the fluidizing principle for metering and transporting powder materials have long been known. Furthermore, it is known that the principle is also used for separating a fluidizable material from a non-fluidizable material. When the fluidizing principle is used to transport oxide, for example, a gas (usually air) is added to the base of a longitudinal, slightly inclined channel in which the oxide is placed by means of the gravitational forces which arise when an oxide flow is released from an oxide feed silo located at a higher location. When the oxide has reached the transport channel and has air blown through it, the oxide is transformed from a sugar/flour-like material to a virtually liquid material in the direction of inclination of the channel and flows forwards along this channel without the use of mechanical transport devices such as belt. The fluidizing principle can be used for all powder materials which can be fluidized, i.e. powder materials which are not mixed with foreign bodies or contain large lumps. In warm regions, where the humidity is high, it is very difficult to get point feeders to work satisfactorily because the oxide easily forms or becomes packed, especially if the oxide contains a lot of fine materials (i.e. is flour-like in its consistency). One way of solving this problem is to add hot air to the oxide as the fluidizing medium in the transport channel, but here the system in itself can constitute a restricting factor.
Norwegian patent application No. 925027 describes a metering device in which the normal fluidizing principle is employed. The problem with such device is that it is not suitable, or not very suitable for use under humid climatic conditions or if the oxide has become damp for other reasons. It is important that a point feeder or a metering device be able to function under the majority of operating conditions.